This invention relates to magnetic resonance image (MRI) imaging, and, more particularly, to methods and apparatus for assembling magnetized permanent magnetic blocks used for an MRI magnetic field generator.
A high uniform magnetic field is useful for using magnetic resonance image (MRI) and nuclear magnetic resonance (NMR) systems as a medical device or a chemical/biological device. At least some popular and low maintenance cost MRI systems currently available use a permanent magnet system that creates a middle range uniform field (0.2 to 0.5 Tesla) in a pre-determined space (imaging volume). A permanent magnet system usually uses multiple permanent magnet blocks such as NdFeB to form a single magnetic object and to achieve desire high uniform magnetic field in the imaging volume.
For a magnetic field generator for an MRI system that uses permanent magnets, the magnets used in such an apparatus are often formulated from a plurality of magnetized blocks. However, it is difficult to place un-magnetized material blocks on a yoke plate first and then magnetize each block. Therefore, in actual manufacturing, the blocks are fabricated and then magnetized. The magnetized blocks are then arranged on a yoke plate so that each of the magnet blocks has a same magnetic pole facing upward. A pole piece is then placed on the top of the magnetized blocks.
It is generally known that when a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MRI magnets typically require not only an intensive uniform magnetic field, typically less than 10 ppm within a 40-50 cm spherical volume, but also an accurate center magnetic field value, typically less than 0.5%. For a given design of the RF coil, the bandwidth is fixed. Thus the allowable range of the center magnetic field is also fixed.
One known configuration of the magnetic field generating device for MRI is the open geometry composed of yokes opposite to each other, connected by post(s), magnetic field generating elements, such as permanent magnets and/or superconducting/resistive coils, and other field shaping components, such as pole pieces.
The magnetic field of the magnet as manufactured, is often influenced by the deviation of material properties, the tolerance of manufacturing process and the environment. For Superconducting and resistive magnets, the center field can be easily adjusted by fine tuning the current. For permanent magnet, however, this is not that easy. Mechanisms typically are built into the magnet in order to adjust the magnet center field after the magnet is finished assembly.